The spine consists of a series of bone structures termed “vertebrae.” Between each vertebra are strong connective tissues termed “discs” which hold one vertebra to another and which also act as a cushion between the vertebrae. The discs are flexible material which absorb shock forces associated with movement. The spine has different sections including the cervical or neck portion, the thoracic chest area and the lumbar lower back or lumbar section. Fusion is a surgical technique in which one or more of the vertebrae of the spine are united or joined to prevent relative movement. The spinal fusion procedure does not directly connect the vertebrae, rather a bone graft is positioned between adjacent vertebrae interbody graft of the spine during surgery. Over a period of time healing occurs as living bone from vertebrae span the interbody graft secures the adjacent vertebrae together. Fusion has occurred when living bone has completely spanned the graft and the adjacent vertebrae are thus connected by a solid bridge of bone.
In the neck, compression of cervical nerve roots by extruded discs or bone spurs is common. Symptoms may include pain, numbness, weakness and disordered reflex symptoms due to compression of a nerve root are called radiculopathy. Cervical radiculopathies are generally treated by resection of the discs or bones utilizing a surgical approach from the front or anterior aspect to the neck. Once the anterior cervical discectomy is completed, common procedure is to place a bone graft between the vertebral bodies in place of the removed disc. Other conditions which may require fusion include treatment of fractured or broken vertebrae, correction of deformities or treatment of instability.
Traditionally, interbody grafts are fashioned from bone taken from a patient's skeleton, termed an “autograft.” Most grafts are now harvested from a cadaver, termed “alografts.” Interbody grafts may also be formed from synthetic materials such as titanium, carbon fiber and plastics. Since harvesting of an autograft is painful, many surgeons prefer the use of alografts. However, alografts are associated with a relatively high rate of dislodgement due to the patient's neck movement during the healing process. To minimize the risk of dislodgement of the interbody graft posteriorly toward the spinal cord, surgeons routinely mortise the graft by drilling a shelf into the vertebrae. To minimize the risk of dislodgement of the interbody graft anteriorly towards the esophagus, surgeons routinely place a metal plate across the inner space and secure it with screws extending into the vertebrae.
Placement of an anterior cervical plate with a screw fixation effectively prevents interbody graft dislodgement toward the esophagus and enhances fusion by providing rigid fixation between the vertebrae. However, this procedure has several disadvantages. In the natural healing process, bone growth and replacement is stimulated by application of force (Wolf's Law). Placement of anterior cervical instrumentation removes forces applied to the neck, potentially lessening replacement bone growth. In addition, if the anterior cervical plate is rigid, it tends to redistribute force asymmetrically to the vertebrae above and below those undergoing fusion. Creation of an unbalanced force may accelerate wear and degenerative changes at the levels adjacent the fusion. Another disadvantage associated with interbody grafts is the loss of vertebral body bone required to create a mortise. Since a typical vertebrae is about 20 mm thick, the creation of a 2 mm mortise results in a loss of approximately 10% of the bone mass available for fusion and requires a fusion mass of greater length to fill the opening created by the dissectomy.
Accordingly, there exists a need for an improved anterior cervical graft and procudure which will eliminate the need to mortise the graft and which will allow compression to facilitate healing and fusion.